A network appliance is a processing system that may be connected to a network to provide service to other devices over the network. A file server, or filer, is an example of a network appliance. A filer is a form of storage server which provides file storage and file access to remote clients that request access to the data storage. Another type of storage server, which may be implemented as an appliance, is one which provides clients with block-level access to stored data, rather than file-level access. Such a server can be used in a storage area network (SAN), for example, to provide an interface between clients and mass storage devices.
Data is stored by a filer in the form of volumes. A volume on a filer may be mirrored onto a volume on another filer. “Mirroring” refers to generating a copy (a “mirror”) of a volume, often on another storage device at a remote location. The copy is a mirrored volume, and can be accessed if the original volume is unavailable for any reason. For example, a first filer may include a volume of data that has been generated by various network users. The volume on the first filer is mirrored onto a mirrored volume of a second filer. The second filer is at a remote location so that if the first filer experiences a service interruption, the second filer probably will not be affected by the cause of the interruption.
Mirroring may be achieved, in part, by writing data coming into the first filer into a memory on the first filer, as well as a memory on the second filer, before writing the data to permanent storage (e.g., a hard drive, optical drive, etc.) The data are stored in the memory until a time known as a consistency point (CP), when the data are written from the memory to the volume on disk.
Mirroring in this manner requires an active connection between the two filers. A link between the two filers is established over a network to facilitate the mirroring relationship. It is possible that the link may be severed without notice to the filers. For example, if the first filer experiences a service interruption, the link between the two filers is no longer active, and the second filer is not notified that the link is inactive. If the filers are unaware of a disruption in the link, the mirroring capability may be lost. What is needed, therefore, is a way to monitor the status of a link between appliances, such as filers, particularly a link used for mirroring data on two or more filers.